1. Field of the Invention
The present invention relates to a coaxial type starter device. More particularly, it relates to an improvement of an overrunning clutch device and a pinion shaft in a coaxial type starter device used for starting an internal combustion engine.
2. Discussion of Background
There has been known a coaxial type starter device provided with a pinion shaft, i.e., a rotary output shaft which is rotated and is movable in its axial direction when it receives a driving force by a motor, disclosed in, for instance, Japanese Unexamined Patent Publication No. 266167/1988.
The coaxial type starter device 1 has a pinion shaft 2 adapted to be rotated and movable in its axial direction wherein a pinion 3 is fixed to the outer circumference of the front end of the pinion shaft 2 by means of a straight spline 4 so as to be engaged with and disengaged from a ring gear attached to the internal combustion engine, as shown in FIG. 4. The pinion shaft 2 passes through the central opening of a clutch inner member 5a, which is an element of an overrunning clutch device 5, and is inserted from its front end in a hollow armature rotary shaft 6a, which is an element of a d.c. motor 6, the pinion shaft having the rear part which is in contact with a pushing rod 8 extending from an electromagnetic switch device 7 interposing a steel ball 9.
A helical spline 2a is formed on the circumference of the pinion shaft 2 placed in the clutch inner member 5a of the overrunning clutch device 5. On the other hand, a helical spline 5b having a length shorter than that of the helical spline 2a is formed in the inner circumference of the front end portion of the clutch inner member 5a so as to be engaged with the helical spline 2a. The rear end portion of the pinion shaft 2 inserted in the hollow armature rotary shaft 6a is supported by a sleeve bearing 10 fitted in the armature rotary shaft 6a in a slidable manner.
In the above-mentioned coaxial type starter device 1, when the electromagnetic switch device 7 is actuated, the pinion shaft 2 is pushed forwardly by means of the pushing rod 8, and the pinion 3 is meshed with the ring gear (not shown) of the engine, and at the same time, the d.c. motor 6 is actuated. Then, the revolution of the armature rotary shaft 6a is decelerated by means of a planet gear type speed reducing device 11, and the rotating force of the armature rotary shaft 6a is transmitted to a clutch outer member 5c of the overrunning clutch device 5. Further, the rotating force of the clutch outer member 5c is transmitted to the clutch inner member 5a through a roller 5d. The revolution of the clutch inner member 5a is transmitted to the pinion shaft 2 through the helical spline 5b, whereby the pinion 3 is rotated. The helical splines 5b, 2a of the clutch inner member 5a and the pinion shaft 2 cooperate to move the pinion shaft 2 in the axial direction to thereby move the pinion 3. They function to transmit a torque produced in the d.c. motor 6 to the pinion shaft 2. Further, they bear a bending force to the pinion shaft, i.e., a radial load which is a component of a force applied to the surfaces of the teeth of the helical splines in the axial direction, said bending force resulting when the pinion 3 is connected to the ring gear. Namely, the helical spline 5b of the clutch inner member 5a and the helical spline 2a of the pinion shaft 2 have a relation as shown in FIG. 5. Specifically, the outer surfaces of the teeth of the helical spline 2a are in contact with the bottom surfaces of the grooves of the helical spline 5b of the clutch inner member 5a, while there is no contact in the other portions. Accordingly, the radial load to the pinion shaft 2 is born by the helical spline 5b of the clutch inner member 5a at the front part, and is born by the sleeve bearing 10 fitted to the armature rotary shaft 6a at the rear part.
As described above, the conventional coaxial type starter device 1 was so designed that the outer surfaces of the teeth of the helical spline 2a are in contact with the bottom surfaces of the grooves of the teeth of the helical spline 5b so that the radial load to the pinion shaft 2 was born by the helical spline 5b of the clutch inner member 5a at the front part. However, the helical spline 2a is generally formed so that the width of the upper surfaces of the teeth are substantially equal to the width of the bottom surfaces of the grooves, and the helical spline is of a helical gear type having equal pitches. Further, since the electromagnetic switch is provided at the rear of the motor in the coaxial type starter device, the length of the helical spline to be formed in the pinion shaft is small because the entire length of the starter device should be as small as possible. Accordingly, the length of the helical spline 5b of the clutch inner member is also small. Accordingly, the surface area in the radial direction of the helical splines 2a, 5b of the pinion shaft 2 and the clutch inner member 5a are small. As a result, the surface pressure becomes large whereby fretting and a shortage of grease often resulted, and a faulty sliding operation occurred in the pinion shaft 2.